The electrical charging system of a typical motor vehicle includes an alternator for generating AC currents, a rectifier to convert these AC currents into a DC current to support the electric loads in the vehicle, as well as to charge the battery. Conventionally, P/N junction diodes are employed as rectifying elements because they provide high current throughput as well as high temperature capability, which makes these diodes well suited for the harsh environment of a motor vehicle.
The forward voltage drop of a P/N junction diode is about 1 volt and the total voltage drop of the rectifier is about 2 volts, resulting in approximately 8–10% of the inputted energy from the alternator being lost in these diodes through heat dissipation. In recent years, the design of Schottky diodes has dramatically improved, such that their current capacity, size, and maximum temperature limit are becoming comparable with P/N junction diodes. Unlike convention P/N junction diodes, Schottky diodes have a much lower forward voltage drop; generally about 45–80% less than a conventional P/N junction diode. Thus, the use of Schottky diodes in a rectifier as rectifying elements can increase alternator efficiency, increase output current at engine idle speed, and reduce heat dissipation.
Unfortunately, the conditions experienced by a vehicle alternator and rectifier limits the ability to use Schottky diodes as rectifying elements. First, Schottky diodes have a maximum allowable reverse voltage that is much lower than conventional P/N junction diodes. At a vehicle load dumping condition, when the vehicle electric load is disconnected instantly, a surge reverse voltage is imposed on the rectifier, which in most vehicles is about 100 volts or higher. The Schottky diodes can be damaged or their characteristics deteriorated after experiencing such a surge reverse voltage. Second, Schottky diodes exhibit a large reverse leakage current mainly at high temperature conditions which are often found in the motor vehicle environment. The reverse leakage currents, depending on their size, could discharge the vehicle battery in a couple of weeks, several days, or even sooner.
Accordingly, there exists a need to provide an alternator and rectifier employing Schottky diodes to increase efficiency and reduce heat dissipation, while at the same time protecting the Schottky diodes from surge reverse voltages and preventing the Schottky diodes from permitting reverse leakage currents.